Empty water, five tons on top (5)
I believe the answer is:
drain
'empty' is the definition.
(draining is a kind of emptying)
'water five tons on top' is the wordplay.
'water' becomes 'rain' (rain is a kind of water).
'five tons' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'on top' means one lot of letters go next to another (in a down clue, some letters go above others).
'rain' after 'd' is 'DRAIN'.
Can you help me to learn more?
(Other definitions for drain that I've seen before include "To empty (of fluid)" , "Wasted down here" , "Nadir (anag.)" , "ditch" , "Siphon off" .)